Push-to-talk (PTT) is a service where users may be connected in either a one-to-one radio communication or in a group radio communication. PTT service has traditionally been used in applications where one person, a “dispatcher” needs to a communicate to a group of people, such as field service personnel like taxi drivers, which is where the “dispatch” name for the service comes from. The PTT functionality is similar to analog walkie-talkies where the users take turns in talking. A user simply presses a button to start transmitting.
Nextel describes its PTT service “Direct Connect” as a “digital, long-range walkie-talkie” built into a wireless handset which lets it “connect” to a similarly configured handset without the need for standard cellular telephone “connect” procedures, such as dialing a telephone number and generating a ringing signal. The conversation exchange is managed as a half-duplex session that allows only one party to talk at a time. Various controls are used to avoid collisions when users transmit at the same time. For example, modern PTT communications often occur via a server, which responds to PTT requests by granting (or denying) a “floor,” i.e., permission to transmit.
A PTT conversation may be communicated over standard, circuit-switched networks as well as over packet-switched, Internet Protocol (IP) networks. Indeed, PTT-type services have been offered on the Internet and are generally known as “voice chat.” These services are usually implemented as personnel computer applications that send vocoder frames in EP packets, i.e., a voice-over-IP (VoIP) service, to a central group chat server, or possibly from client-to-client in a peer-to-peer service.
Push-to-talk calls are desirable because they use bandwidth efficiently—an important benefit in radio communications where radio bandwidth is a scarce and expensive resource. They also permit group calls as easily as one-to-one calls. Another key advantage of PTT services is that the communication is quick and spontaneous, initiated by simply pressing the PTT button, without a going through typical dialing and ringing sequences.
Unfortunately, set-up times and propagation delays in PTT communications diminish the interactive “feel” when the push-to-talk communication is two-way, e.g., A and B are having a conversation. The set-up time for a digital PTT call may take two to three seconds. An example initial PTT request-response cycle might look as follows:    At zero seconds, user A pushes a button to initiate a PTT conversation with user B and talks for ten seconds.    At three seconds, the set-up is finished, and user B starts playing out the information sent by A.    At thirteen seconds, the initial message from A is completely played out by B. At that very same moment, (assuming that B has extremely quick reactions), B pushes its PTT button to respond and talks for three seconds.    Sometime after sixteen seconds, (there are transfer delays in the system), A starts playing out B's response message.As a result of the time associated with setting up the initial PTT connection between A and B and with communicating the first message from A to B, there is a significant delay after A stops talking and before A receives B's response message. This delay is quite noticeable, and although it may have been acceptable in one-way dispatch communications, such delays diminish the real time feel of two-way, interactive PTT calls. Indeed, these kinds of delays are troublesome in any kind of real time, interactive communication.